1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to cleaning devices for streamers used in marine seismic surveying and, more particularly, to autonomous cleaning devices and related methods for cleaning marine growth and other contaminants deposited on the streamers towed underwater.
2. Discussion of the Background
Marine seismic surveying investigates and maps the structure and character of geological formations under a body of water using reflection seismology. Reflection seismology is a method of geophysical exploration especially helpful in the oil and gas industry. In marine reflection seismology, the depth and the horizontal location of features causing reflections of seismic waves are evaluated by measuring the time it takes for the seismic wave to travel to receivers. These features may be associated with subterranean hydrocarbon reservoirs.
A typical marine seismic surveying system is illustrated in FIG. 1. A vessel 100 tows a seismic source 102 and plural streamers such as 106, each streamer carrying an array of seismic receivers 104 (e.g., hydrophones). It is desirable to maintain the streamers at predetermined horizontal cross-line distances (i.e., along an axis perpendicular to the towing direction T), and at predetermined depths (e.g., 10 m) relative to the water surface 108. The seismic source 102 is configured to generate a seismic wave 110 that propagates downward (down, up and vertical being defined relative to gravity) toward the seafloor 120 and penetrates the formations 125 under the seafloor 120 until it is eventually reflected at discontinuity locations such as 122a and 122b. The reflected seismic waves 130a and 130b propagate upwardly and can be detected by one of the receivers 104 on the streamer 106. Based on the data collected by the receivers 104, an image of the subsurface formation is generated by further analyses of the collected data.
To maintain the streamers at a desired position (i.e., such as to have predetermined cross-line distances and predetermined depths), conventionally, a horizontal position control device 140 is attached to the streamer, most frequently at the front of the streamer 106, close to the towing vessel 100. Separate conventional depth control devices 145 (e.g., floaters) may be employed to maintain a depth of the streamer.
Streamer Rescue Devices (SRDs) 150, which are configured to help recover portions of streamers that have been accidentally severed, are placed typically at regular intervals along the streamers (e.g., every 300 m for a streamer of about 12 km). An SRD is activated to generate signals when the respective portion of the streamer sinks below a predetermined depth (e.g., 48 m) and/or pressure increases above a predetermined level (e.g., 70 psi).
Significant amounts of bio-fouling settlement accumulate on the exterior surface of the streamers. These accumulations can obscure the reflected seismic wave and significantly increase the streamer drag. The rate of accumulation and the impact of the bio-fouling and other contaminants depend on factors, among others, such as the geographic location, the water temperature, and the season. The gooseneck barnacle is the most common bio-fouling organism found on the marine streamers.
Cleaning such contaminants from the exterior of the streamers is desirable and beneficial. For example, a cleaning device 160 with brushes or other cleaning tools may be moved along the streamer to clean contaminants from the exterior thereof.
A conventional cleaning device, such as the one disclosed in U.S. Pat. No. 7,754,018, typically includes one or more cleaning elements (e.g., brushes) and a mechanism configured to attach and to roll the cleaning device along the streamer in order to clean the exterior of thereof. The cleaning device may include buoyant bodies. The frame of the cleaning device may be designed to pass over the SRDs and the depth control devices while moving along the streamer (e.g., the frame may have a center clearance).
A conventional cleaning device 165 described in U.S. Pat. No. 7,145,833 and illustrated in FIG. 2 has two vanes (e.g., 170a and 170b) attached to a cylindrical body 175 formed by two half cylinders 177 engaged on one side by hinges 179 and on another by any suitable releasable fastener (not shown). Brushes 180 are attached inside the body 175 via inserts 185 to reach the outer surface of the streamer 106. The vanes are angled with respect to longitudinal axis of the body 175 to provide both thrust and torque to the body 175.
Most conventional cleaning devices are passive devices that are moved relative to the streamer by the water flow. They are deployed at the front of the streamer (i.e., close to the towing vessel) and picked up at the end of the streamer, after a cleaning operation along the streamer. The cleaning devices are then manually repositioned for another cleaning operation along the same streamer or another streamer. Human intervention in retrieving and repositioning the cleaning devices is a source of inefficiency and renders the cleaning of the streamer expensive.
An active cleaning device is described in U.S. Pat. No. 7,409,919. This cleaning device includes a turbine rotated by the water flow caused by the movement of the streamer through the water, and a drive element configured to convert the rotation of the turbine into motion of the cleaning device along the seismic streamer. The drive element includes wheels or other mechanisms that are in contact with and moving relative to the streamer. In order to reverse the motion from upstream (i.e., in the towing direction) to downstream (i.e., in a direction opposite to the towing direction) the pitch of the turbine blades is changed. This cleaning device has the disadvantage that the turbine and the drive train to wheels can be damaged by seaweed or fish line entangled on the moving parts dragged through the water. Additionally, since the drive elements are constantly in tight frictional contact (e.g., loaded by spring suspension) with the streamer, it may at times be difficult for the cleaning device to roll over the contaminants deposited on the streamer's outer surface, and it occasionally may be even impossible to pass obstacles with larger diameter, such as, weights or bird collar overmolds that are often seen along streamer sections.
In recent years, new positioning devices for streamers (named “birds”) have been developed. The birds are inserted between streamer section ends, along the streamer and are able to develop forces to adjust both a depth and a horizontal location. For example, U.S. Pat. No. 7,610,871 discloses a bird having three fins attached to an outer shell of the bird's body. The fins are mounted to individually rotate around their respective transverse (i.e., perpendicular to the towing direction) axes of rotation. One fin, which normally has its rotation axis along gravity, is free to rotate and is ballasted or linked to a ballast, performing a roll and/or orientation stabilization of the bird while in motion. The other two fins are controlled to rotate around a respective rotation axis at individual rotation angles, so as to develop desired horizontal and vertical forces while dragged through the water, steering the bird toward an intended position of the streamer.
While being superior to older positioning devices for streamers, these birds are sometime also bulkier. The conventional cleaning devices are rendered obsolete, because the birds are too large for the cleaning devices to pass over them. Therefore, there is a need to develop cleaning devices for streamers designed to operate when birds or other obstacles are present on the streamers.